[ntp:questions] NMEA ref.clock better than my ISP's timeserver?

Unruh unruh-spam at physics.ubc.ca
Fri Jun 12 08:24:31 UTC 2009


David Lord <snews at lordynet.org> writes:

>David J Taylor wrote:
>> David J Taylor wrote:
>> []
>>> I might be inclined to go for a screened
>>> cable if you need a 30m run, but thin 3-core mains cable /might/
>>> suffice.

>I'm intending buffering the pps to give 75r output to coax with
>another converter back to ttl at the server. The NMEA should manage
>the distance over twisted pair at 4800 baud.

The nmea is virtually useless for accurate timing. The main thing that
the unit gives you is the PPS. You have to makes sure you do not degrade
it. 



>> 
>> I'm not sure whether the NMEA driver attempts to send anything /to/ the 

Which NMEA driver? 
>> GPS device.  If not, three lines might to (ground, TX from GPS, PPS), 
>> otherwise four lines are required.  If you want to take USB power from 
>> the server (as I now do), it's five lines minimum.

>I'd rather have the option for two way in case the Garmin needs to be
>set to a different mode. I have a reel of utp I think should do.

>I'll have some sort of fan-out box to get a pps signal to each of
>servers that are powered up continuously.

>I've now seen an error in ntp log and suspect pps isn't enabled in
>kernel by default (NetBSD-5). I'll check tomorrow.

Run the PPS to say gpsd or shmpps and then use ntp to use it to
discipline the clock. PUtting the PPS into the kenrel has no advantages
as far as I can see. 





>Cheers

>David




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